As is well known, image formation conducted by an electro-photographic process incorporates the process in which a toner image is formed via electrostatic charge, exposure and development, the formed toner image is transferred onto a recording sheet and the toner image transferred onto the sheet is fixed to form a permanent image.
In this fixing process, when toner is melted by heat and pressure to fix the image onto the sheet, some water evaporates from a paper sheet due to the heat. Further, after the fixing, the sheet is open to the outside air, and thereby reabsorbs water from the outside air.
Since such evaporation and re-absorbance of water between the front and reverse surfaces of the paper sheet tends to occur at a different rate, it results in waving or curling of the sheet, which of course is a major problem.
Generally, a fixed paper sheet shows various phenomena. FIG. 1(a) shows wavy sheet S3 in which the printed surface is convexly and concavely curled. FIG. 1(b) shows a sheet which is convexly curled upward, which hereinafter is referred to as “convexly curled sheet S1”, while FIG. 1(b) shows a sheet in which the printed surface is convexly curled downward, which hereinafter referred to as “concavely curled sheet S2”. The conveyance direction of the sheet is shown by an arrow in FIG. 1(a), 1(b) and 1(c).
These phenomena generate various problems. In the case of post-processes conducted on the sheet which carries a formed image, problems occur during conveyance and processing of the sheet, and in the case of binding process, a book of bound curled sheets becomes very thick, which result in deterioration in quality of the final product. Further, when such sheets are ejected from the apparatus, they do not stack orderly. Accordingly, measures to count such wavy and curled sheets are required.
In Unexamined Japanese Patent Application Publication 61-23,068, to overcome the problem of the wavy or curled sheets, the sheet is moisturized via a mist of water. A moisturizing apparatus incorporating a moisturizing roller is shown, to effectively control the water supply, to reduce any adverse effects due to the water mist on various sections adjacent to the moisturizing apparatus, and to downsize the apparatus.
Moisturizing flattens wavy and curled sheet, but it curls the sheet again if water is not applied equally to both surfaces of the sheet.
Specifically, if more water is applied onto one surface than the other surface, the former expands more than the latter, and a convex curl is generated on wetter surface.
In U.S. Pat. No. 6,052,553, a moisturizing apparatus is shown in which moisturizing rollers are provided on and under the horizontal conveyance route through which the sheet is fed. The upper and lower moisturizing rollers apply water to both surfaces of the sheet. In this apparatus, water is supplied to the upper and lower moisturizing rollers from respective water tanks.
In the case water is applied onto both surfaces of the sheet, due to the water routes from the surface of the water tank, serving as the water source, to the contact position of the moisturizing roller and the sheet, the upper moisturizing means and the lower moisturizing means are not equal, which makes it very difficult to apply equal amounts of water onto both surfaces of the sheet.
Even if water is equally applied onto both surfaces by adjustment, the supplied water amount changes during operation, which causes curled sheets. Further, in above-mentioned moisturizing apparatus, more than three rollers are required for the water supply route, which result in a larger apparatus.
In Unexamined Japanese Patent Application Publication 2006-8,282, a moisturizing apparatus is proposed in which paired moisturizing rollers convey the sheet horizontally, and apply water onto both surfaces of the sheet. A water tank, provided under the lower moisturizing roller, supplies water to the lower moisturizing roller, but the upper moisturizing roller is via the lower moisturizing roller.
In this apparatus, the applied water amount from the upper roller to the sheet tends to be lower than that of the lower roller, which causes the curled sheet.
In these apparatuses, the sheet is conveyed horizontally as it is moisturized. As shown in FIG. 2, since water applied to the sheet moves vertically due to gravity, the amount of water changes in accordance with the direction of thickness, and curling tends to be generated. In FIG. 2, moisture particles in sheet S are schematically shown by solid black dots w.